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The Murphy Corner-Line-Array
An Open Loudspeaker Design Project

by John L. Murphy
Physicist/Audio Engineer
1. MCLA Project Home
2. Design Concepts
3. MCLA Project Details
4. Single Driver Test Results
5. Array Test Results
6. Line References/Links
7. Project Log
8. MCLA Discussion Thread

 1. MCLA Project Home

The MCLA

Project Overview

The Murphy Corner-Line-Array (MCLA) is a new type of DIY line array loudspeaker system that I have designed to meet my own requirements for reference monitor loudspeakers with sufficient output  to also serve as the sound reinforcement system for my band. I am pleased to make this novel loudspeaker design freely available for everyone who wishes to enjoy this new level of quality in sound reproduction. I hope to provide sufficient detail here for DIY speaker enthusiasts to reproduce the system for their own enjoyment and/or profit.

PUBLIC DISCLOSURE OF INVENTION
Title of the Invention: Murphy Corner-Line-Array Loudspeaker
Inventor: John L. Murphy
Physicist/Audio Engineer
Andersonville, TN USA

Summary of the Invention:

The corner line array loudspeaker consists of a line array loudspeaker designed specifically to optimize the placement of the speaker's reflected images when placed in the corner of a room. More precisely, the enclosure is designed to fit in the corner in such a way as to smoothly merge the real enclosure with its reflections into the acoustic equivalent of an octagonal enclosure located in free space. See Figure 1.1 below for the details.


Figure 1.1: The Murphy Corner-Line-Array As Viewed From Above

The "corner-line-array" loudspeaker constitutes an improvement over previous point, line and planar loudspeakers by combining the reflected acoustic images with the direct sound radiation in such a way as to effectively create an infinitely long line source radiator where the side wall and front wall reflections of the line are uniquely located in very close proximity to the real speaker system by virtue of the corner placement. When viewed from above along with its reflections, the unique corner-fitting shape is seen to join with its reflected images to form an overall octagonal shaped line source with loudspeakers on four sides.

As a direct result of controlling the precise reflection placement, the in-room frequency response that is achieved by the corner-line-array loudspeaker system is more consistent as the listener moves about the room than previous types of loudspeaker systems. It is also more consistent from room to room than other types of loudspeakers. When equalized to have a flat frequency response or other targeted response (such as the X-curve) the Murphy Corner-Line-Array provides a consistently accurate frequency response about much of the listening room and from room to room. Previous loudspeakers experienced highly variable frequency response because of the lack of standardized placement in the room and the resulting variable and widely spaced room reflections.

This invention is being disclosed publically in order to deliberately and immediately place the invention fully in the public domain for the benefit of all.

John L. Murphy
9July2009

(a signed and witnessed disclosure is on file at True Audio)

Having been fully described here in the public domain since 9July 2009,  my "corner-line-array" invention cannot now be claimed or patented by any third party. As far as I am concerned, everyone is free to use it as they wish with no strings attached. I humbly request only that you openly acknowledge the origin and name of the design.

As an audio product designer for the past 30 years I am not easy to please when it comes to loudspeakers. I've heard them all and have definite opinions about what I want when it comes to sound reproduction. The role of these speakers will be to serve as reference monitors in both my home studio and my home theater. I recommend this system wherever a high performance sound playback system is required. My recommended applications include: two-channel home music playback systems, home theater (especially the front corners!) and small sound reinforcement systems for small rooms. There is one basic requirement for the room: The room must have about an 8 foot ceiling which is parallel to the floor with two corners available for placement of the line arrays. If your listening room conforms to this description then I expect the MCLA will perform in your room very much like it performs in my rooms. This should allow you to benefit from my testing and fine tuning as I voice the system.

Each array employs 24 identical full range drivers in a floor-to-ceiling enclosure specially designed to take full advantage of corner placement. After searching the available drivers I have selected the new Dayton Audio ND90-8. This is a 3.5" driver with solid aluminum cone and 4mm of linear excursion (peak). 

A Little History

I first came to admire line array type loudspeakers around 1979 when I was still simulating enclosure responses on a hand-held calculator…point-by-point and plotting them by hand. Now, after having lived with a large (16 cu ft!) pair of line arrays over the last 30 years I have designed this new corner-line-array speaker system for use in both my home studio and my home theater. You are invited to review my design and my measured performance and perhaps you will decide to construct a pair for your own use.

My original line arrays (which I designed and built in 1980) were a two-way system employing eight 8” woofers and sixteen 1” soft dome tweeters. The crossover varied between active and passive types through the years but always hovered in the 1.5 kHz range. Butterworth, Linkwitz-Riley filter types…I’ve tried them all. Regardless of the crossover details I wanted to keep the crossover frequency as low as possible for optimum off-axis frequency response in the crossover range. By design, the incredibly high power handling capability of the tweeter array allows them to be safely crossed at a much lower frequency than if using only a single driver. Over the years these two line-array systems have performed marvelously and typically make a strong positive impression on all who have auditioned them. They have wonderfully low distortion even at the highest sound levels a body can tolerate. Many of the audio industry insiders who auditioned my line arrays back in the 1980's had never heard high performance line arrays before. Today I see concert line array loudspeakers offered by at least one company whose founders could have been seen leaving my place with a smile after hearing the MLA 8/16 system back in the 80's, long before concert line-arrays came into popular use.

From the start these two-way line arrays employed active (line level) equalization in various forms. Initially it took the form of a fixed equalizer which was designed based on in-room measurements of the actual system response. Eventually I settled on an off the shelf one-third-octave equalizer for flexibility in voicing the system’s frequency response. The responses I keep handy in the EQ’s memories include (echoic in-room) “flat”, “X-curve” and “half X-curve”.  The flat response is reserved for very select recordings which have been recorded with flat microphones and mixed and mastered on flat audio monitoring systems. My typical setting is what I consider to be the norm for most recorded music and film audio: the X-curve. Now with my beloved "MLA 8/16" line-arrays approaching 30 years in age I have developed the MCLA full-range corner-line-array in a more compact enclosure to replace them as my master reference monitors.

Appendix 1: The original MLA 8/16 line array system (circa 1980)

Design Overview

A: Line Array Source Geometry

Based on 30 years of listening to my previous line array speakers the new system could only be another line array.

B: Crossover-Free Full-Range Operation:

The array employs a high quality full-range transducer of small diameter but large excursion capability. The exact native frequency response of the driver is less important than the ability to be equalized to cover from 30 Hz to 20 kHz (-3 dB points). I first elected to use a 3 to 4 inch transducer based on my long term experience with a 4 inch full range speaker I enjoy in one room at my home. This speaker convinced me that there is no clearer midrange than that of a crossover-free full-range speaker. Splitting the frequency band into two (or more) parts and then recombining them via separate different sized speakers inevitably introduces audible coloration. The 2.5 inch piston of the full-range driver may have narrower high frequency dispersion than a typical 1 inch dome tweeter but the geometry of the corner-line-array actually prevents the listener from ever being more than 45 degrees off axis. Thus the requirement for wide dispersion is reduced compared to a speaker located in the room that might be heard at 90 or even 180 degrees off axis. The dispersion of the 2.5 inch piston is certainly wide enough considering that the MCLA listening angle is limited to 45 degrees. Ultimately, the very best crossover...is NO crossover. The power amplifier is connected directly to the full range drivers with no passive components in-between.

C: Corner Loading:

By designing the system specifically for corner placement the side and front wall reflections are forced into close proximity to the real speaker. As expected from corner loading, the multiple images increase low frequency output capability because of the close proximity of the images to the real speaker. This standardized placement in the room eliminates the major source of variations in the frequency response as the listening position changes within the room. It also minimizes the difference from room to room. This standardized room placement actually gives everyone a good chance of achieving a system response very close to my own documented response by reproducing the line arrays exactly and then using my carefully constructed equalization settings.

D: Extended Bass to below 30 Hz (-3 dB)

The arrays are recommended as full range loudspeakers for both home audio and home theater applications. A subwoofer is not required. When used as a small room sound reinforcement system in live music applications the goal is to provide bass extension to 50 Hz. Restricting the bass extension for sound reinforcement applications will allow for an overall higher SPL level before any limit is reached. I deliberately elected a tradeoff of lower efficiency for a smaller sized enclosure that would provide bass response to around 100 Hz. This means that below 100 Hz the system has increased power requirements compared to a non-equalized system. Given the tremendous efficiency of the overall system however, my 100 Watt (at 8 Ohms) power amp seems adequate for my applications. So I am currently using a power amp that is rated far below the actual power handling of the system. (20 Watts/driver = 480 Watts per system). Even though they need EQ to get bass extension the arrays are still operating with a generous amount of headroom. This keeps distortion very low compared to other speakers which typically operate at much higher individual driver stress levels.

E: High SPL Capability for Sound Reinforcement Applications

I want the system to be capable of serving as the PA system for my band when we perform in my studio. For this application it is reasonable to limit the bass response to around 50 Hz (-3 dB) to reduce the excursion requirement and allow for the increased overall system SPL required for live music. With the ability to perform sound reinforcement for a live band, they are just loafing when playing recorded music at loud home playback levels. So far I have actually been using the 30 Hz mode without any signs of stress during our band rehearsals. I suspect that in a larger room the 50 Hz limit would be needed to avoid problems.

F: Active Line Level Equalization:

The system employs a line-level equalizer in order to achieve various target responses including (echoic in-room) “flat”, X-curve and Small Room X-curve. Equalization is currently achieved by an off the shelf 1/3 octave digital equalizer: the Behringer Ultra-Curve Pro DEQ2496. But I may also design a custom analog equalizer to replace the digital EQ and lower the total cost of the project. Stay tuned...

Driver Selection

The performance of the full-range driver is critical for the project. The characteristics of the driver which are most important are upper frequency extension, distortion and excursion capability (both Xmax and maximum mechanical excursion). The driver has to be correctable to 20 kHz and must have enough linear volume displacement such that 24 drivers will provide adequate output to 30 Hz. Surprisingly, the F(s), Q(ts) and V(as) parameters, and even the box volume are not critical for this project as long as they are "reasonable" because the frequency response will be equalized to flat and the closed box design "transformed" to a new second order response with the desired Q and F3.

The driver I have selected for the project is Dayton Audio's new ND90-8. This is a high performance 3.5" driver with aluminum cone and 4mm of Xmax (peak). The driver is being custom manufactured to Dayton Audio's specifications by one of their OEM sources and this new design takes full advantage of Dayton's long and broad experience in driver design. Dayton currently has a large supply of ND90 drivers on hand and the supply should be ongoing as long as there is demand for the driver. I have followed the development of the driver and have been testing and auditioning samples beginning in March 2009.

As one of my listening tests I routinely used a single ND90 test box as the speaker for my electric guitar during personal practice sessions. Playing my Telecaster's single coil pickups through my Quad X guitar preamp followed by a power amp and ND90 at close range provides an intimate musical connection that is very enjoyable. My ear tells me that this is a very low distortion speaker when driven within its power handling range. Using just a single driver it is possible to explore how the driver sounds when it is overdriven without getting terribly loud. If I turn up my guitar (keeping it squeaky clean at the preamp) I can hear exactly how the ND90 sounds as it overdrives. I was pleased to hear that it distorts softly with no surprises and even though I definitely abused it I have never heard it bottom out on the bass as many drivers would do in this situation. This is likely due to the very generous maximum excursion capability of 10mm (peak). This allows a full 6mm of headroom beyond the 4mm (peak) linear excursion. It's nice to know the driver will overdrive gracefully when pushed to its limits even though the full array will never be pushed that hard in actual use. To drive the complete array into distortion would require a much larger power amp than I plan to use and would produce dangerously high sound levels. At the loudest sound levels I can tolerate the individual drivers will be loafing. That's the power advantage of an array.

The availability in recent years of small diameter drivers (3 or 4 inches) with low distortion and high excursion makes this kind of loudspeaker system feasible. While I will be standardizing the project on the ND90 note that other 3" to 3.5" drivers with adequate performance could be used to create a high performance corner-line-array. Upon surveying the small number of qualifying drivers I have elected to specify the Dayton ND90 for the project.

Here is a link to the ND90 and some photos:

Dayton Audio ND90-8 at Parts Express


Figure 1.2: The Dayton Audio ND90 Full Range Driver

Variations on the Design

At this time I am documenting here one very specific implementation of the MCLA for the DIY loudspeaker community. But, in general, the corner-line-array design can be implemented with broad variations in components and scale. Certainly the array might be built with different full-range drivers or as a multi-way system with different types of transducers. The number of drivers used and the total length of the array could be varied to suit many different applications. Smaller clubs and theaters could certainly take advantage of corner-line-arrays.

Corner arrays for rooms with ceiling heights other than eight feet come to mind immediately. It would be simple to increase the number of drivers to span a ten foot height and then come up with a wiring arrangement to achieve a net impedance around 8 Ohms. The overall size of the enclosure could be increased or decreased to vary the native bass response or explore the performance of even smaller full-range drivers. While equalization can be used to achieve extended bass response from small drivers, I have made it a point to have a generous degree of headroom in the "24xND90" system detailed here but is is reasonable that others might explore implementations of arrays of smaller drivers of less output capability. Scaling the enclosure down would tighten up the spacing of the corner reflections further.

While my prototype enclosures are implemented in wood the enclosure might also be made as a two piece aluminum or plastic extrusion. An extrusion would allow the enclosure to be shaped as a quarter-round with a flat area for mounting the driver. The effective shape of the enclosure plus its reflections could be made even closer to a perfect cylinder with edges more rounded than my "octagon".

Arrays of typical dynamic transducers, such as the ND90, will always require frequency response correction in order to neutralize the inherent 3 dB per octave slope (falling with increasing frequency) that is characteristic of long line arrays using many drivers. The equalizer used to perform the frequency response correction could be either digital or analog, custom or off-the-shelf.

 

Next: 2. Design Concepts

 

Project Documentation and Discussion
The 12 page MCLA Project Brief:    MCLA Project Brief.pdf
revised 6Jul10
The complete 51 page MCLA project document:     MCLA Project.pdf
revised: 16Jul10
MCLA discussion at the Parts Express Tech Talk Forum:  MCLA Discussion Thread

 

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